248 research outputs found
The Galactic halo magnetic field revisited
Recently, Sun et al. (2008) published new Galactic 3D-models of magnetic
fields in the disk and halo of the Milky Way and the distribution of cosmic-ray
electron density by taking into account the thermal electron density model
NE2001 by Cordes & Lazio (2002, 2003). The models successfully reproduce
observed continuum and polarization all-sky maps and the distribution of
rotation measures of extragalactic sources across the sky. However, the model
parameters obtained for the Galactic halo, although reproducing the
observations, seem physically unreasonable: the magnetic field needs to be
significantly stronger in the Galactic halo than in the plane and the
cosmic-ray distribution must be truncated at about 1 kpc to avoid excessive
synchrotron emission from the halo. The reason for these unrealistic parameters
was the low scale-height of the warm thermal gas of about 1 kpc adapted in the
NE2001 model. However, this scale-height seemed well settled by numerous
investigations. Recently, the scale-height of the warm gas in the Galaxy was
revised by Gaensler et al. (2008) to about 1.8 kpc, by showing that the 1 kpc
scale-height results from a systematic bias in the analysis of pulsar data.
This implies a higher thermal electron density in the Galactic halo, which in
turn reduces the halo magnetic field strength to account for the observed
rotation measures of extragalactic sources. We slightly modified the NE2001
model for the new scale-height and revised the Sun et al. (2008) model
parameters accordingly: the strength of the regular halo magnetic field is now
2 microG or lower, and the physically unrealistic cutoff in z for the
cosmic-ray electron density is removed. The simulations based on the revised
3D-models reproduce all-sky observations as before.Comment: 11 pages, 6 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
The DRAO 26-m Large Scale Polarization Survey at 1.41 GHz
The Effelsberg telescope as well as the DRAO synthesis telescope are
currently surveying the Galactic polarized emission at 21 cm in detail. These
new surveys reveal an unexpected richness of small-scale structures in the
polarized sky. However, observations made with synthesis or single-dish
telescopes are not on absolute intensity scales and therefore lack information
about the large-scale distribution of polarized emission to a different degree.
Until now, absolutely calibrated polarization data from the Leiden/Dwingeloo
polarization surveys are used to recover the missing spatial information.
However, these surveys cannot meet the requirements of the recent survey
projects regarding sampling and noise and new polarization observation were
initiated to complement the Leiden/Dwingeloo Survey. In this paper we will
outline the observation and report on the progress for a new polarization
survey of the northern sky with the 26-m telescope of the DRAO.Comment: 5 pages, 6 figure
A statistical analysis of a Galactic all sky survey at 1.4 GHz
Radio surveys at frequencies of about 1 GHz allow to map the synchrotron
emission in a frequency range where (except for very low Galactic latitudes or
towards localized regions) it dominates over the other radio components. New
all sky total intensity and polarization data at 1.4 GHz have been recently
collected. We focus on the Galactic radio emission correlation properties
described in terms of angular power spectrum (APS). We present for the first
time the APS, in both total intensity and polarization modes, for some
representative Galactic cuts and suitable APS power law parametrizations.Comment: Comments: 2 pages, 2 figures; in Astronomische Nachrichten, Vol.327,
Issue 5/6, p.491 (2006); Proceedings of International Conference "The Origin
and Evolution of Cosmic Magnetism", 29 August - 2 September 2005, CNR Area
della Ricerca, Bologna, Italy, eds. R. Beck, G. Brunetti, L. Feretti, and B.
Gaensle
Rotation Measure Synthesis of Galactic Polarized Emission with the DRAO 26-m Telescope
Radio polarimetry at decimetre wavelengths is the principal source of
information on the Galactic magnetic field. The diffuse polarized emission is
strongly influenced by Faraday rotation in the magneto-ionic medium and
rotation measure is the prime quantity of interest, implying that all Stokes
parameters must be measured over wide frequency bands with many frequency
channels. The DRAO 26-m Telescope has been equipped with a wideband feed, a
polarization transducer to deliver both hands of circular polarization, and a
receiver, all operating from 1277 to 1762 MHz. Half-power beamwidth is between
40 and 30 arcminutes. A digital FPGA spectrometer, based on commercially
available components, produces all Stokes parameters in 2048 frequency channels
over a 485-MHz bandwidth. Signals are digitized to 8 bits and a Fast Fourier
Transform is applied to each data stream. Stokes parameters are then generated
in each frequency channel. This instrument is in use at DRAO for a Northern sky
polarization survey. Observations consist of scans up and down the Meridian at
a drive rate of 0.9 degree per minute to give complete coverage of the sky
between declinations -30 degree and 90 degree. This paper presents a complete
description of the receiver and data acquisition system. Only a small fraction
of the frequency band of operation is allocated for radio astronomy, and about
20 percent of the data are lost to interference. The first 8 percent of data
from the survey are used for a proof-of-concept study, which has led to the
first application of Rotation Measure Synthesis to the diffuse Galactic
emission obtained with a single-antenna telescope. We find rotation measure
values for the diffuse emission as high as approximately 100 rad per square
metre, much higher than recorded in earlier work.Comment: Accepted for publication in The Astronomical Journa
The local Galactic magnetic field in the direction of Geminga
The Milagro hot spot A, close to the Galactic anticenter direction, has been
tentatively attributed to cosmic rays from a local reservoir (at a distance
~100 pc), freely streaming along diverging and smooth magnetic field lines.
This is at variance with the geometry of the ~kpc scale Galactic magnetic
field, which is known to be aligned with the spiral arms. We investigate the
information available on the geometry of the magnetic field on the scales (~100
pc) of relevance here. The magnetic field immediately upstream of the
heliosphere has been investigated by previous authors by modeling the
interaction of this field with the solar wind. At larger distances, we use the
dispersion measure and the rotation measure of nearby pulsars (especially
towards the third Galactic quadrant). Additional information about the local
field towards the North Polar Spur is taken from previous studies of the
diffuse radio emission and the polarization of starlight. The asymmetry of the
heliosphere with respect to the incoming interstellar medium implies a magnetic
field almost orthogonal to the local spiral arm, in the general direction of
hot spot A, but more to the south. This is in good agreement with the nearby
pulsar data on the one side, and the North Polar Spur data on the other. The
local magnetic field on scales of ~100 parsecs around the Sun seems to be
oriented so as to provide a direct connection between the Solar system and a
possible site of the Geminga supernova; the residual angular difference and the
shape and orientation of the Milagro hot spot can be attributed to the field
trailing in the wake of the heliosphere.Comment: 5 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Faraday caustics: Singularities in the Faraday spectrum and their utility as probes of magnetic field properties
We describe singularities in the distribution of polarized intensity as a
function of Faraday depth (i.e. the Faraday spectrum) caused by line-of-sight
(LOS) magnetic field reversals. We call these features Faraday caustics because
of their similarity to optical caustics. They appear as sharply peaked and
asymmetric profiles in the Faraday spectrum, that have a tail that extends to
one side. The direction in which the tail extends depends on the way in which
the LOS magnetic field reversal occurs (either changing from oncoming to
retreating or vice versa). We describe how Faraday caustics will form
three-dimensional surfaces that relate to boundaries between regions where the
LOS magnetic field has opposite polarity. We present examples from simulations
of the predicted polarized synchrotron emission from the Milky Way. We derive
either the probability or luminosity distribution of Faraday caustics produced
in a Gaussian magnetic field distribution as a function of their strength, F,
and find that for strong Faraday caustics P(F)\proptoF^{-3} . If fully
resolved, this distribution is also shown to depend on the Taylor microscale,
which relates to the largest scale over which dissipation is important in a
turbulent flow.Comment: 14 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
A multifrequency angular power spectrum analysis of the Leiden polarization surveys
The Galactic synchrotron emission is expected to be the most relevant source
of astrophysical contamination in cosmic microwave background polarization
measurements, at least at frequencies 30'. We
present a multifrequency analysis of the Leiden surveys, linear polarization
surveys covering essentially the Northern Celestial Hemisphere at five
frequencies between 408 MHz and 1411 MHz. By implementing specific
interpolation methods to deal with these irregularly sampled data, we produced
maps of the polarized diffuse Galactic radio emission with pixel size of 0.92
deg. We derived the angular power spectrum (APS) (PI, E, and B modes) of the
synchrotron dominated radio emission as function of the multipole, l. We
considered the whole covered region and some patches at different Galactic
latitudes. By fitting the APS in terms of power laws (C_l = k l^a), we found
spectral indices that steepen with increasing frequency: from a = -(1-1.5) at
408 MHz to a = -(2-3) at 1411 MHz for 10 < l < 100 and from a = -0.7 to a =
-1.5 for lower multipoles (the exact values depending on the considered sky
region and polarization mode). The bulk of this steepening can be interpreted
in terms of Faraday depolarization effects. We then considered the APS at
various fixed multipoles and its frequency dependence. Using the APSs of the
Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the
rotation measure, RM, in the simple case of an interstellar medium slab model.
Taking also into account the polarization degree at 1.4 GHz, we could break the
degeneracy between the identified RM intervals. The most reasonable of them
turned out to be RM = 9-17 rad/m^2.Comment: 18 pages, 14 figures. Astronomy and Astrophysics, in pres
The magnetic field of the Large Magellanic Cloud revealed through Faraday rotation
We have measured the Faraday rotation toward a large sample of polarized
radio sources behind the Large Magellanic Cloud (LMC), to determine the
structure of this galaxy's magnetic field. The magnetic field of the LMC
consists of a coherent axisymmetric spiral of field strength ~1 microgauss.
Strong fluctuations in the magnetic field are also seen, on small (<0.5
parsecs) and large (~100 parsecs) scales. The significant bursts of recent star
formation and supernova activity in the LMC argue against standard dynamo
theory, adding to the growing evidence for rapid field amplification in
galaxies.Comment: 15 pages, including 3 embedded EPS figures (1 color, 2 b/w) plus
supporting on-line material; uses scicite.sty. To appear in Science, vol 307,
number 5715 (11 March 2005
Measuring and calibrating Galactic synchrotron emission
Our position inside the Galaxy requires all-sky surveys to reveal its
large-scale properties. The zero-level calibration of all-sky surveys differs
from standard 'relative' measurements, where a source is measured in respect to
its surroundings. All-sky surveys aim to include emission structures of all
angular scales exceeding their angular resolution including isotropic emission
components. Synchrotron radiation is the dominating emission process in the
Galaxy up to frequencies of a few GHz, where numerous ground based surveys of
the total intensity up to 1.4 GHz exist. Its polarization properties were just
recently mapped for the entire sky at 1.4 GHz. All-sky total intensity and
linear polarization maps from WMAP for frequencies of 23 GHz and higher became
available and complement existing sky maps. Galactic plane surveys have higher
angular resolution using large single-dish or synthesis telescopes. Polarized
diffuse emission shows structures with no relation to total intensity emission
resulting from Faraday rotation effects in the interstellar medium. The
interpretation of these polarization structures critically depends on a correct
setting of the absolute zero-level in Stokes U and Q.Comment: 10 pages, 8 figures. To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
Polarization surveys of the Galaxy
We report on sensitive 21cm and 11cm polarization surveys of the Galactic
plane carried out with the Effelsberg 100-m telescope at arcmin angular
resolution and some related work. Highly structured polarized emission is seen
along the Galactic plane as well as up to very high Galactic latitudes. These
observations reflect Faraday effects in the interstellar medium. Polarized
foreground and background components along the line of sight, modified by
Faraday rotation and depolarization, add in a complex way. The amplitudes of
polarized emission features are highly frequency dependent. Small-scale
components decrease in amplitude rapidly with increasing frequency. We stress
the need for sensitive absolutely calibrated polarization data. These are
essential for baseline setting and a correct interpretation of small-scale
structures. Absolutely calibrated data are also needed to estimate the
high-frequency polarized background. A recent study of polarized emission
observed across the local Taurus-Auriga molecular cloud complexes indicates
excessive synchrotron emission within a few hundred parsecs. These results
suggest that possibly a large fraction of the Galactic high latitude total
intensity and polarized emission is of local origin.Comment: 6 pages with 2 PS figures. To be published in "Astrophysical
Polarized Backgrounds", eds. S. Cecchini, S. Cortiglioni, R. Sault and C.
Sbarra, AIP Conf. Pro
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